The present invention relates generally to a seal assembly for a fluid pump, and to a method for detecting leaks in the fluid pump or the seal assembly. In particular, the present invention relates to a seal assembly for use in a magnetic-drive centrifugal pump, and a method for detecting leaks in the containment shell of a magnetic-drive centrifugal pump.
Background art fluid pumps may externally leak pumped fluids upon the failure of a fluid containment device or a seal. Because pumped fluids may contain caustic, hazardous, toxic, or flammable chemicals, the external leaking of pumped fluids from fluid pumps may lead to dangerous conditions for workers in industrial processing plants. The prompt identification and remedying of pumped fluid leaks minimizes environmental and safety dangers.
The containment of hazardous fluids depends on the integrity of fluid containment devices, such as seals, containment shells, gaskets, mechanical seals, packing, or the like. Containment devices may leak spontaneously because of flow rates and head values which significantly vary from optimum pump operating conditions (i.e. best efficiency point) or from the pump manufacturer's recommended operating conditions. For example, if a pump is operated at a excess capacity, or where the flow rate is greater than recommended at a corresponding head, the pump will surge and vibrate causing potential shaft seal problems. Similarly, if a pump is operated at reduced capacity, potential seal problems are caused by increased radial thrust of the shaft and increased vibration.
Fluid containment devices may fail because of a prodigious variety of environmental factors. In particular, pressure and temperature changes in the pumped fluid may contribute to the failure of fluid containment devices. For instance, pump seals or containment shells may fail because an industrial worker manually adjusts a valve, which results in a sudden pressure shift at the pump. Likewise, pump seals or containment shells may fail because of disruptive or abrupt flow rate changes initiated by automatic controllers. In manufacturing processes, where storage vessels are coupled to pumps, damaging pressure changes may be caused by level changes of fluids stored in the storage vessels.
Temperature changes may be caused by the ineffective operation of heat exchangers coupled to the pump or ambient temperature changes. Excessive heat can cause gasket and seal materials, such as elastomers, to become brittle and inflexible, so that sealing efficiency is degraded. In addition, heat can cause thermal distortion of gasket materials and elastomers so that sealing efficiency is degraded.
Problems with containment devices in centrifugal sealless pumps are particularly intractable because the thickness and resultant strength of the containment shell are restricted by the inherent design of the centrifugal sealless pump. Centrifugal sealless pumps include two varieties: canned sealless centrifugal pumps and magnetic-drive sealless centrifugal pumps. In a canned sealless centrifugal pump, metallic containment shells are deliberately kept thin to reduce electrical eddy currents and corresponding thermal problems. Even where nonmetallic containment shells are used the thickness of the containment shell is limited by the requisite proximity for efficient magnetic field interaction of the stator and the rotor.
In magnetic-drive sealless centrifugal pumps, a first magnet is coupled to a second magnet (or a torque ring) through a thin containment shell. In magnetic-drive pumps, the containment shell generally has a thickness not exceeding three millimeters. The containment shell is typically spaced less than one millimeter from the first magnet and less than one millimeter from the second magnet. Nonmetallic containment shells have a thickness limited by the requisite proximity for efficient magnetic coupling of the first magnet and the second magnet. Consequently, the relatively thin containment shell may be corroded or damaged due to the chemical, thermal, or caustic attributes of the pumped fluid. In addition, the containment shell may be damaged by pressure surges occurring, for example, when a user opens or closes a valve in pipes coupled to the fluid pump.
To determine the integrity of fluid containment devices, users of pumps typically disassemble pumps and inspect fluid containment devices at regular time intervals. However, a containment device can fail virtually instantaneously; and even immediately after the soundness of the fluid containment device was previously verified. Hence, identifying the failure of sealing devices by disassembly of pumps, is time-consuming and impractical.
Therefore, a need exists for a seal assembly that can readily verify the integrity of the seal assembly or containment shell of a fluid pump without disassembly of the fluid pump. In addition, the need exists for a centrifugal pump which can offer additional reliability, improved hazardous pumped fluid containment, and reduced pumped fluid emissions compared to background art pumps.